In the widely practiced Exxon melt-blowing process, molten resin, for example molten polypropylene, is extruded through a row of linearly disposed holes of diameter about 0.35 to 0.5 mm. The holes are drilled in linear array on about 1 to 2 mm centers into a flat surface about 1 to 2 mm wide, with the surface being located as shown in FIG. 1 at the apex of a member having a triangular cross section, and with the angles at the apex being about 45.degree. to 60.degree. to the center line. Surrounding the apex 11 as shown in FIG. 1 are two slots 12, 13, one on each side, through which is delivered heated air, which attenuates the molten resin extruded through the holes, thereby forming a stream of fibers. The fibers are collected on one side of a moving screen which is separated from the nozzle tips by about 10 or more cm, with the other side of the screen being connected to a suction blower. In operation, most of the fibers are collected on the screen to form a low density web with a rough surface; however, a significant proportion of the fibers escape into the surroundings, and a suction hood into which they are collected and sent to waste is provided.
The collected web is quite weak, with tensile strengths well below about 1.5 kg/cm.sup.2 ; for example, a 0.06 cm thick web would fail at an applied force of less than 0.09 kg/cm of width, and elongation prior to failure is less than about 5 to 10%. The fibers have a wide size distribution, with the largest fibers being ten or more times larger than the smallest, and the average fiber diameter is about five to seven or more times the smallest fiber diameter. Many of the fibers are twinned, with a twin being defined as two parallel fibers adhering to each other along a length of 20 or more times their average diameters, while others are roped, i.e., consist of two or more fibers twisted about each other in a form resembling a rope. Roped fibers behave in practice, for example in filtration, much like a single fiber of diameter about equal to that of the rope. Both twinning and roping cause the collected web to have a high pressure drop and low filtration efficiency. Fibers bonded in a twin are less efficient in a filter than two separate fibers. Shot--i.e., small pellets of unfiberized resin interspersed in the web--are also a problem. As stated in U.S. Pat. No. 3,825,380, one of the Exxon patents: "The Naval Research Laboratory work covered die-nose included angles of 30.degree. to almost .pi..degree. with 60.degree. recommended as the best compromise between making shot and rope." The products of the present invention are by contrast substantially free of shot and roping. The rough, rather fuzzy surface of the Exxon system web is undesirable for many applications, for example, for use in disposable clothing.
The Exxon system is inefficient by virtue of its geometry. When the two air streams converge, a portion of the energy required to form fibers of the resin is dissipated in proportion to the component of their velocities perpendicular to the center line of the apparatus. A further inefficiency is the rectangular shape of the air stream which acts on each nozzle; if 0.5 mm diameter holes are located on 2 mm centers, a rectangular air stream 2 mm wide acts on each 0.5 mm diameter resin passageway. Because the liquid stream is circular, that portion of the air issuing from a corner of the rectangle farthest from the resin nozzle is relatively ineffective, generating a high degree of turbulence with a relatively small contribution to fiber formation. As a result of these inefficiencies, the cost of energy to compress and heat the air in the Exxon process is much larger than it would be if each resin nozzle were to receive its own supply of air through a circular annulus. Due to the high volume of air required to fiberize a given weight of resin, the distance from the resin nozzle outlet to the fiber collection surface in usual practice exceeds about 10 to 13 cm, and this relatively long passage through very turbulent air causes the undesirable roping and twinning in the fibers of the collected web. Attempting to operate at much less than 10 cm makes collection on the vacuum screen difficult, unless the fibers are so hot as to be semi-molten, which produces a near to solid product which is inefficient as a filter. In a basic deficiency of the Exxon system, the molten resin is disrupted external to the fiberizing die and is simultaneously attenuated to form fibers; there is no clear delineation between disruption and fiber formation, and as a result control of fiber formation is poor.
The present invention employs self-contained individual fiberizing nozzles comprising an annular air passage. These fiberizing nozzles have been used to make commercially available filter elements tradenamed "HDC" (Pall Corporation, East Hills, N.Y.), are capable of making fibrous sheet media with average fiber diameters as small as about 3.5 .mu.m, with a lower limit of about 3.0 .mu.m, and are operated at a die-to-collector distance (hereinafter DCD) of about 5.5 to 9 cm. The present invention also comprises recently developed novel self-contained fiberizing nozzles capable of making fibrous sheet media with average fiber diameters less than about 1.0 .mu.m, are operated in the DCD range of about 2.8 to 9.0 cm, and make product with controlled orientation of the fibers.
Such a fiberizing nozzle is depicted in FIG. 2, wherein the fiberizing nozzle 21 contains a capillary 22 through which the resin is pumped and a circular annulus 23 through which hot air is delivered. The pumped resin exits the capillary 22 into the resin disruption zone 24 and then into the nozzle channel 25 where the resin, now fragmented into tiny droplets, is carried in the air stream out of the nozzle tip 26.
Because the air supply is used more efficiently and is correspondingly less in proportion to the weight of the product web, the fiberized product of the present invention can be collected as a web by impinging it on a solid collecting surface, as opposed to the vacuum backed screen of the Exxon apparatus. In another marked improvement on the prior art, the DCD (distance between the nozzle tip 26 in FIG. 2 and the target collecting surface) may be shortened to under about 2.8 to 5.5 cm, i.e., about one half or less than used for the Exxon system, thereby reducing the width of the fiber stream and further improving fiber collection efficiency.
U.S. Pat. No. 4,021,281 describes a method in which attenuated fibers are produced by a die not materially different from the Exxon dies, in which the molten resin is disrupted externally to the die. The fibers produced are described in column 8, lines 48-53, as "collected in a randomly oriented heterogeneous intertwined arrangement on the mandrel, since virtually no control is exercised over the path the fibers follow in their trajectory from the spinning die to the mandrel. By the time the fibers reach the mandrel, they are either already broken up or disrupted into discontinuous lengths, or they are still attached to the orifice from which they are spun by a portion which is molten. In the latter case, the fiber is continuous."
Contrasted with the above, the fibers of the instant invention are formed within the fiberizer nozzle, and can be seen by direct observation through a microscope to be fully formed and not in contact with the orifice out of which the fiber stream passes. The absence in the invention of disruption external to the orifice is essential to the formation of a web in which the fibers are continuous and distinctly oriented in a controlled fashion.
Thus the fibers of the present invention are generated, controlled, and collected in a manner which produces a web of oriented fibers, contrasted with, as stated in column 8, lines 48-52, of U.S. Pat. No. 4,021,281, a heterogeneous intertwined arrangement, with no control over the path of the fibers.
The collection system of U.S. Pat. No. 4,021,281 illustrated in FIG. 1 of that patent is generally not capable of making product webs of the uniformity currently required for use in diagnostic devices, which are used in small pieces, for example as narrow as 0.5 cm wide. In part this is due to relatively poorer control of the fiber deposition and in part because, as may be seen in FIG. 1 of the patent, there is a considerable length of fiber between 36 and 37, which tends to stretch and contract lengthwise during operation, thereby creating zones which contain too much or too little fiber.
While finer fibers and improved collection are made possible by the use of individual nozzles, their use has the disadvantage that the product web has a stripy appearance. The stripes reflect the spacing between adjacent nozzles. Despite this disadvantage the products of this system have proved themselves in a wide variety of applications, for example in applications such as the filtration of liquid foods, in which a single layer of the filter medium is often used.
The present invention provides a convenient means to collect the output of individual fiberizing nozzles in the form of a web which is not only substantially free of striping, but is characterized by a high degree of uniformity, for example by weight distribution varying less than about 1% over a fifty cm span. Such a degree of uniformity makes the product useful in critical applications such as diagnostic devices, as well as for other applications where near to perfect uniformity is required.